1. Field of the Invention
The present invention relates to a continuous flow rotary pump, preferably a continues axial flow rotary pump for impelling liquid through at least one stage, by transferring energy from rotating elements of the pump to a continuous fluid stream, and more preferably the invention relates to a continuous axial flow rotary pump for use in blood circulation assistance, either in intravascular or extravascular circuits, with maximized efficiency and with no, or at least extremely minimized, blood damage, blood clotting, as well as minimum pump dimensions.
Although particular reference will be made in the present specification to a blood pump, it should be understood that the present pump is for use in any other field wherein any fluid must be transferred from one place to another one, either in a closed circulation loop or in any open circuit or path.
2. Description of the Prior Art
It is well known to provide an axial-flow rotary pump comprising a generically cylindrical casing and/or stator with a rotor, or a plurality of rotors mounted inside the stator to drive a fluid through the pump. The driving of the liquid to transfer the same from an inlet of the pump to a pump outlet is based in the provision of energy to the liquid to increase the fluid pressure thereof. This energy, however, provides several undesired side effects. The elimination of these effects without impairing the pumping efficiency of the pump has been the aim of many developments in the field of pumps, particularly when handling of sensitive fluids, such as explosives, blood, etc., is involved.
Contours, sizes, assemblies and relative positions of the different parts, as well as the stationary and movable surfaces of a pump are aspects and parameters that must be defined when designing the pump. The final objective of the design is to get a maximum efficiency of the pump with a minimum or no side effects resulting from the energy transferred to the fluid during the impelling thereof. Particularly in the case of a blood pump design, the aim is to reach to a pump having a maximum efficiency without side effects causing blood damage and/or blood clotting during operation. Another important objective is to have a pump having a minimum size.
The side effects resulting from the energy transferred during rotation of the pump comprise the generation of secondary or side flows, vortex, cavitation and separation of the flow from the surfaces of the stationary and movable parts of the pump.
The continuous fluid flow behavior through a rotary pump provided with blades is mathematically defined by the Euler equation. According to Euler, pressure energy imparted by the rotor is proportional to the increment of the tangential component of velocity. Analysis of the Euler equation is made through the so called velocity triangles shown in FIG. 1 for a conventional scheme. Vectors represent averaged velocities on a flow surface and the letter references used in FIG. 1 are:
.omega. angular speed R radius u = .omega..R rotation velocity C absolute velocity W relative velocity C.sub.u tangential component of absolute velocity
index 1 is used for the pump inlet
index 2 is used for the pump outlet
The Euler equation applied to a conventional rotary pump is: ##EQU1##
This is the reason why traditional pump designs include stator blades at the pump outlet, thus trying to reduce as much as possible the tangential component of the velocity and transform the kinetic energy into pressure energy.
Although many efforts have been made to eliminate or at least reduce the above mentioned side effects, by reducing or eliminating the above tangential component, for example, no solutions have been found hereinbefore. When a small Reynold's number is involved, that is when one handles small pumps and/or viscous liquids, stator blades at the pump outlet can not effectively reduce the tangential component of the velocity and transform kinetic energy into pressure energy, no matter the shape or number of blades provided. Therefore, flow separation and side flows are formed at the stator blades which cause hemolysis and blood clotting.
There are indeed several patents disclosing pumps with stator blades at the pump outlet with the purpose of eliminating, as much as possible, the tangential component of fluid speed exiting the impelling stage of the pump. U.S. Pat. No. 4,846,152, issued to Richard K. Wampler, discloses a miniature intravascular blood-pump formed as a single stage with a rotor and an elongated stator, the rotor having two rows of blades and the stator having a single row of blades, within a tubular housing. The blades of the stator are reversed-twisted and have an unusual length to straightens and slow the blood flow so as to prevent the deposit of blood particles. This stator, however, does not provide for the elimination of any tangential component of the flow speed at the exit of the pump.
U.S. Pat. No. 4,908,012 to John C. Moise, discloses an implantable ventricular assistance pump having a tube in which a pump rotor and stator are coaxially contained, and purge fluid is introduced into stator blades of the pump to avoid creation of discontinuities in the blood path wall. The object of this cited patent is to reduce the size of the implant and minimize the risk of infection by reducing vibration, minimizing the percutaneous conduit, and directing most of the heat generated by the pump into the blood. The problem of the flow kinetic energy is not addressed and, in fact, the provision of the bladed stator does not reduce the tangential component of the flow speed.
U.S. Pat. No. 5,209,650 to Guy B. Lemieux, discloses a pump integral with an electric motor and impeller assembly that rotates within a stator casing and is supported on hydrostatic radial and thrust bearings so as to avoid having to provide external seals or friction type bearings. Although rotors rotating in opposite directions are provided in this patent, it is clearly disclosed in its specification that the invention addresses the problems that occur with leaking mechanical seals and worn bearings. While Lemieux specifically includes stay vanes pitched to diffuse the liquid from the second stage integral rotor and impeller assembly, the problem of kinetic energy and tangential components of the blood flow is not considered, and it can not be overcome in any way by providing, as disclosed and illustrated in this patent, axial rotors separated by axial stators.
U.S. Pat. No. 5,211,546 to Milton S. Issacson discloses an axial flow blood pump including stator blades and rotor, the object of which is to minimize the structure by which the rotor is suspended with respect to the stator to minimize the overall diameter of the motor and pump combination. No considerations are made relating to the tangential components of the blood flow and the side effects resulting thereof.
U.S. Pat. No. 5,588,812 to Lynn P. Taylor discloses an implantable electric blood pump having a motor stator and a rotor, the stator including blades for slowing and de-spinning the blood flow.
U.S. Pat. No. 5,678,306 to Richard J. Bozeman discloses a method for optimizing each of a plurality of blood pump configuration parameters in the known pump components and variations. While Bozeman includes a diffuser with five to eight fixed blades for deaccelerating and redirecting the outflow at blood flow path exit to boost pump performance, the problem of the tangential components of the speed is not solved.
U.S. Pat. No. 5,707,218 issued to Timothy R. Maher discloses an axial-flow blood pump having a rotor suspended in ball-and-cup bearings which are blood-cooled but not actively blood-lubricated. While, Maher includes outlet stator blades for slowing and de-spinning the blood flow for discharge into the pump outlet, again, the problem of the tangential components in the blood flow is not addressed.
Other rotary pumps are known from U.S. Pat. Nos. 4,779,614; 5,040,944; 5,112,292 and 5,692,882 but these documents have not addressed the problem of the tangential component of the flow velocity.
Concluding, the problem of the flow separation and secondary flows have not been addressed and solved by any of the patents mentioned above.
It would be therefore convenient to have a continuous axial-flow pump having a minimum quantity of components and capable of providing a continuous flow without side effects resulting from the kinetic energy of the circulating fluid and affecting the fluid integrity, particularly to avoid the blood damage and blood clotting by eliminating the flow separation and secondary flows.